Raman spectroscopy is an analytical technique that has a tremendous variety of potential uses. Among other desirable characteristics, Raman spectroscopy is compatible with aqueous media and does not typically require sample preparation. The technique is particularly attractive due to its suitability for remote analysis via optical fiber. By employing optical fibers as light conduits, the light source and light detector can be physically separated from the sample. This remote attribute is particularly valuable in process control and especially in sensing and analysis involving harsh chemicals in hostile environments.
In a typical configuration for fiber-optic-based Raman analysis, one or more illumination fibers deliver light from a source to the sample. The light source is typically a laser, and this type of analysis is often referred to as a branch of fiber-optic-based laser spectroscopy. Upon interaction with the sample, the laser light is scattered into distinct wavelengths that differ from the laser wavelength. One or more collection fibers receive the scattered light from the sample and transmit it to a detector. The characteristics of the source light are compared to that of the received light. Two characteristics are particularly important. The wavelength separations between the laser light and the bands of scattered light are specific to the chemicals within the sample. The spectral intensity of the scattered light is a function of the sample's chemical concentration.
The Raman scattering effect is extremely weak. Only a small fraction of the excitation light is Raman scattered. Because the signals are weak, the probe's delivery of light to and collection of light from the sample must be highly efficient. And, the introduction of extraneous signals severely corrupts the measurement quality.
When sampling fluid media, the sample to be analyzed is often flowing within a pipeline or is turbulent in a reactor vessel. The medium under inspection is often dark or exhibits other aspects that complicate the measurement.
In process control environments, the conditions are often so hostile as to necessitate physical and chemical isolation of the probe's optical fibers from the surrounding environment. To accomplish such isolation, a probe may incorporate a window behind which its fibers are positioned. The incorporation of a window into a probe introduces a significant engineering problem. As emitted illumination light passes through the window and into the sample, a portion of this light is back reflected by the window's inner and outer surfaces. In the prior art, this undesired back reflected light is inadvertently introduced into the collection fibers along with the desired Raman scattered light. The back reflected light corrupts the quality of the analysis. Light inadvertently introduced into a photonic instrument is often referred to as stray light.
The issue of stray light in fiber optic probes for Raman spectroscopy is complicated by another factor. As laser light propagates through the fiber from the source to the sample, the light interacts with the fiber core and is scattered. Fiber scattering effects may include fiber fluorescence, Raman scattering, and other interference. This fiber-scattered light will be referred to as silica-Raman light, but is not exclusive to silica fibers or to the Raman effect. The longer the optical fiber, the more intense the silica-Raman light. Thus, the light that is back reflected off the probe window contains silica-Raman light in addition to the primary laser light. This silica-Raman light is particularly troublesome to the measurement as it is spread over broad wavelengths. Once mixed with the desired Raman light from the sample and introduced into the collection fiber, the desired light cannot be easily isolated from the silica-Raman light. The problem is further complicated because back reflected laser light, which is inadvertently received by the collection fibers, also generates silica-Raman light as it propagates from the probe to the detector.
The prior art includes a variety of attempts to address the problems discussed above.
U.S. Pat. No. 5,166,756 to McGee et al. describes a probe for analyzing powders. In this probe, a multiplicity of illumination and collection fibers are arranged behind a sapphire rod. The rod's end face is inclined relative to the optical fibers' end faces. In accordance with the patent's teaching, back reflections from the rod's outer surface are angularly oriented outside of the collection fibers' reception capabilities. In this manner, the subject can be analyzed with reduced interference from window reflection.
The probe described in McGee et al. suffers from several drawbacks. The window's thickness removes the fibers a significant distance from the sample, which results in decreased efficiency. The overlap between the illumination zone and the collection fibers' field of view is not precisely controlled. This also contributes to poor efficiency and requires the use of finely stranded fiber optic bundles, which also suffer from many drawbacks. The probe's optical characteristics are dependent on the position of the illumination and collection fibers relative to the window's outer surface and to one another. Maintaining repeatability of these factors is difficult in fabricating the probe. Therefore, probe-to-probe performance repeatability, particularly as it relates to broad band intensity, suffers.
Similar problems plague related methodologies that angularly orient various aspects of a window such that the window's planar surfaces are not perpendicular to the fiber's longitudinal axis. For example, U.S. Pat. No. 4,573,761 to McLachlan et al. describes a probe in which the optical axis of an illumination fiber and the axes of multiple collection fibers are directed into intersection by bending the collection fibers near their ends. From their positions behind a window, the collection fibers receive back reflections from the window. These reflections greatly reduce measurement quality.
U.S. Pat. No. 5,402,508 to O'Rourke et al. describes a probe that employs shaped end faces on parallel fibers to improve optical efficiency. Refraction at the fiber's end face bends the fibers' illumination and viewing fields toward overlapping regions. Although this concept appeared to be promising, it suffers from several limitations.
Although O'Rourke et al. teach that reflections from a window's outer surface are not troublesome for deployment in liquids, the opposite is often the case. Windows fabricated from strong, chemically resistant materials, such as sapphire and diamond, have refractive indices that are much higher than most solutions. For example, sapphire's refractive index is approximately 1.77, while water's refractive index is about 1.33. This refractive index differential results in the reflection of light exiting the window. Lower refractive index window materials such as silica can be employed. However, because these materials are typically much weaker and less chemically resistant, their usage requires a thick window. The increased window thickness results in increased back reflection and decreased collection efficiency.
A sample's refractive index can vary depending on many factors, including temperature and composition. The refractive index often changes independently of the parameter that the analysis seeks to isolate. Therefore, the stray light cannot be easily removed by compensation.
In gaseous media such as air, the problem is particularly acute. Although usage of thin windows minimizes the collection of back reflections arising from a window's outer surface, thin window attachment to the probe housing is difficult and the resulting assembly is mechanically weak.
The preferred embodiment described by O'Rourke et al. depicts the illumination and collection fibers as being separated by a gap. This type of separation can be utilized to minimize the collection of window-based back reflection. However, the separation between illumination and collection fibers results in poor photonic efficiency.
In another prior art approach, probes were formed by encircling a flat-faced illumination fiber with a ring of flat-faced collection fibers. The resulting fiber optic bundle was then positioned behind a window. The illumination fiber emits light through the window and into a sample. The collection fibers receive light scattered by the sample. When the illumination and collection fibers are flat, the optical axes of the emission and reception patterns do not intersect and the probe is inefficient. In addition, the probe suffers from stray light arising from back reflection of illumination light from the window's outer surface.
Improved performance was accomplished by shaping the end of the group of collection fibers, while leaving the center illumination fiber flat. In particular, the probe's center fiber end face remains flat, and the surrounding fibers' end faces are angled and formed on a taper. This results in a taper between the bundle's outer cylindrical surface and the center fiber's flat end face. The refractive effect of the shaped end faces causes the collective field of view of the collection fibers to converge on the optical axis of the illumination fiber. The overlap between the illumination fiber's emission zone and the collection fibers' reception zone, which occurs in the sampled medium, is established fairly close to the probe end face and, when compared to flat-faced probes, results in increased photonic efficiency, especially in dark, absorbing media. However, even with all of its desirable characteristics, this type of probe is prone to stray light from back reflections of emitted light off the window.
A variety of filtering techniques have also been employed in various attempts to overcome the described problems. However, their usage involves many engineering challenges and application-related limitations.
Therefore, there is a need in the art for a probe that minimizes stray light resulting from window reflections while concurrently providing controlled overlap between the illumination fibers' emission fields and the collection fibers' reception fields.